Arduino UNO Project

Hy there,

I am kinda new here and I want to start a new project based on Arduino UNO. I want to control 3 leds or 3 relays( I don't know so far) from my computer via USB->Arduino UNO. I have a C program that when runs write a value in a .txt file. That value I want to be like a command for the uC: what led should turn ON.

Its something I can do with Arduino UNO?

Arduino can not read files from the PC, you must write any program for PC to sendi commands via serial line (COMx) to Arduino. can I do that?